Amplifier



Patented June 9, 1942 Nils E. Lindenblad, Port Jefferson, N. Y., assigner to Radio Corporation of America, a corporation of Delaware Application July y17, 1937, Serial No. 154,152, vwhich is a division of application Serial No.

4,474, February 1, 41935.

Divided and thi! IID- pucsuon April 29, 1939, serial Namaste 4 claims. (c1. 17a- 171) My prior Patent No. 2,131,566, granted September 27, 1938, relates to amplier ofthe thermionic type and is in particular concerned with an amplifier of flat characteristics over an ex'- tremely wide frequency range such as, for example, 20 to 1,000,000 cycles. yThe amplifier ofy the said invention makes use of thermionic tubes now in commercial use, although ,certain characteristics of said tubes, such asl limited electron emission and large inter-electrode capacity, work frequencyband. Both capacities are to' varied extents operative'v at frequencies intermediate the-extreme ranges. @The amplier described above has been claimed in the above mentioned patent and'described in divisional application #154,152, led Julyy 17, vv'1937, now Patent N0.

2,224,915, December 17,' 1940, ofwhich application the present is a further division.

g As indicated above, the amplifier of the present against the operation of said tubes effectively in known circuits over a frequency band of less Width than the frequency' band involved in this invention. By my novel circuit, a frequency band of extreme width is successfully amplified, and all of the frequencies in said band are amplified substantially the same amount. f

In working on this problem, it at once became clear that transformer' coupling' between the stages must be excluded at least for fthe time being, because of the limited frequency range which can be passed by transformer coupling means. I next investigated the possibilities of capacityresistance coupling. Moreover, it vwas found desirable to use such coupling in conjunction with push-pull type of amplifier circuits due to their symmetry and the ease with which neutralizing may be accomplished'in such circuits.

termine such coupling constants which would result in a fiat amplifying characteristic between to 1,000,000VV cycles. It became apparent that a condenser with a high coupling capacity and having the necessary insulation to separate the plate voltage in one stage from the grid voltage.

in the next stage is of necessity alco'ndenser of appreciable physical dimensions. A condenser of such dimensions has an appreciable stray capacity, while this stray capacity has only a small shunting effect at 20 cycles, its shunting effect at 1 megacyclehas a paralyzing iniuence on the efforts to obtain th'e desired flat characteristic. On theother hand, if a condenser of smaller physical dimensions be used, it cannot be given enough capacity to carry'through the lower frequencies.l 4It also became clear that a compromise could hardly be 'obtained and that insted of a compromise, some kind of combination ofthe two condensershad to be worked out in yorder to obtain the desired result. The solution that was finally arrived at Awas to combine one small and one large capacity as components in a network or impedance trap of such a characteristic that practically only one of the two capaci'- Having` decided on these points, the task left was to depliiier described, above to connectionv withl single-ended s ource.

Vinventionis, for reasons also pointed out above,

of the push-pull type with added means for adapting the push-pull wide frequency band am- 'Ihe nature of my invention and the manner in which the same is` carried out will be understood'froin the following detaileddescriptionv ties are in function at each extreme end of the thereof and'therefor when read in connection with the drawing throughout which like characters indicate like parts insofar as possible and invvhich:` f

The drawing illustrates a thermionic amplifier of the push-pull type comprising several interconnected stages in which a novel'means is ineluded for feeding the push-pull* amplifier from the output of a.v single tube or single-ended stage lwhile the output circuit yof the final stage is arranged to feed either a push-pull utilization circuit .or a single-ended circuit.

In the figure I have shownan amplifier of the push-pull output type, comprising pairs of electron 'discharge tubes- 2, l, 2'. 4', connected in cascade by way of resistance and capacity networks, as shown. The input'of the first stage is vconnected to asinsle-ended stage by means to be described in moredetail later. 'I'he anode-tocontrol grid capacity of each stage is as shown neutralized by a capacity NC. The output 'ci' the push-pull amplifier may be connected to a push- `pull utilization circuit bymeans' of leads represented by the solid line or to a single-ended -input by means of one of the said leads and a llead. represented by the dotted line.

As will be seen by an inspection of the figure, the small capacities C1 are-directly connected between the anodes of the tubes 2, 4 and the control grids of the following tubes 2. l'. respectively, whereas the'large capacities yCi are as shown connected in parallel to the capacity C1 by way of link resistors R1. The succeeding stages if used are connected in a similar manner. f

' During operation at the high frequency end of the band the small capacities C1 have a low reactance and therefore theV coupling current prefers passage through these capacities rather e Ca by way of the resistors R1. This might seem inenicient to force the coupling current through a resistance, but it must be remembered that...l

the tube reactance being extremely high at these frequencies results in a very high grid impedance, since the grids are biased to carry` no electron current. The fact that the load'` from the grids is negligible at the low frequencies therefore makes the fact insignificant that they are fed through resistors R1. By actual measurements of the characteristics of this circuit, it

'as shown. These condensers, if used must be amplitude equal to the amplitude of the voltages on the grid of tube 2 and of a phase opposite in phaseto the voltages on thegrid of tube 2 may be applied to `the grid of tube l so that we have a true push-pull inputs' The 'source Y provides a counter-electromotive force to insure correct 1direct-V currentvoltage on the grid of vtube l. Condensers proportional, in reactance, to the resistance on eachside of the tapping point may be connected with the resistance X very small.4 An additional circuit containing a was found that it was not necessary to take too great precautions in keeping the stray capacities of condensers C: down by providing very spacious shields S around the said capacities. The spacing of the shields was made smaller and this is ldesirable as it resultsin smaller dimensions of the amplier Vas a whole. The stray capacities of these condensers not only'can be made larger, but it was found advisable and necessary to do so. To increase theV stray capacities of the condensers C2, I provide as shown, an additional condenser Ca which is preferably variable. This condenser, which is small in capacity relative to condenser C2, may be connected'on either side of Ca between Ciand ground. In this manner, a fine regulation of the stray capacity may be accomplished. Another reason fortheneed of the variable capacity C; is that at the intermediate frequencies of about 100,000 cycles, the amplifier without theuse of \said capacities gives somewhat higher amplification due to an optimum impedance condition 'of my simple network without a correct stray capacity of condenser Cz. With this condition corrected by means of adjusting'the stray capacity, the variation lin ampliilcation throughout the whole band between 2'0 cycles `and 1 megacycle was at onetime 'brought below two decibels (1.2521).

In operation, it is desirable to have a well regulated or steady direct current source B supplying plate current to this type of amplifier, as shown in the parent application above referred -'Ihe connections whereby my amplifier may point on said resistanceand thecontrol grid of o the other tube of said push-pull stage and a rebe connected to single-ended circuits in such a way that theffpush-pull stages receive balanced input'will now be described.

Assume that the signal is to be supplied from the output of a single-ended stage betweenthe controlgrids of one of the tubes 2, 4 and groundA as shown by way of the leads marked Input; th`e signal on the gridof tube 2 modulates or controls the voltage on the plate of tube 2 and this control or variation is opposite in phase to the controlling .potentials applied to the grid.y This voltageyvariation or modulation appears across the resistor X connectedv as shown by wayof source Yfbetween the plate of tube 2 and ground. Voltage variations or modulations are supplied fromthe resistance or potentiometer X by way of a movable point' and a lead to the control grid of tube 4. By movingthe point on the potentiometerX to the proper position, voltages of any resistor X' may be added to the plate 'of tube l to complete the balance. v I claim:

single ended stage on the control grids of a pair of therinionic` tubes in push-pull fashion in like amounts comprising a conductor arranged to be connected to the output electrode of said single endedstage and tothe controlvgrid of one of said tubesaresistance connected to the anode of said one tube and to ground by way of a source ofpotential, a connectionsbetween` a movable point on said resistance and the Acontrol grid of the other tube of said push-pull stage vand small Vcondensers connected from said Vmovable point to each end of said resistance, the capacitive impedance ratio between said condensers being proportioned to the ratio ofresistances on eachside of said movable point.

singleI ended stage onthe controlgrids of a pair of thermionic tubes in push-pull fashion in like amounts comprising a conductor larranged to be connected tothe output electrode of said single ended stage and to the control grid of one of said tubes, a resistance connected to the anode of said one tube and to groundby way `of a source of potential, a direct connection between a movable sistance connected from the anode or the other of said `tubes to ground the second of said re-l sistances being Vequal in 'vfalue to the first;

3. Means for impressing oscillations from a single ended stage on thefcontrol grids of a pair connected to the output electrode of said single ended stage and to 'the control grid of one of said tubes, a resistancey connected to the .anode of said one tube and to ground by way of a source of potential, a connection between a movable pointv on v`said resistance yand :the control grid of the other tube of said push-pull stage and a resistance and a condenser in shunt thereto connected from the anodeof 4the other of said tubesi to ground.

` `4. Means for impressingv oscillations Afrom a single ended stage on the control grids of ,a pair of thermionic tubesin push-pull fashion in like 4amounts comprising-a conductor arranged to Vbe connected to the output electrode of said single ended stage and to the kcontrol grid of one of saidtubes, a' resistance connectedto the anode of said yone tube and to ground by wayy of a source of potential, a yconnection betweenk a movablev pointson said resistanceand the control grid of the other tube of said push-pull stage and small condensersconnec'te'd from .said movable point to each end of said resistance anda resistanceV kand a condenser in shunt thereto connected, from the anode of the other of said tubes to ground. NILs E. IJNDENBIAD. 

